1. Field of the Invention
The present invention relates to high-frequency semiconductor devices. The present invention more particularly relates to a high-frequency field effect transistor (FET) used for a millimetric-wave or quasimillimetric-wave circuit module for amplification, oscillation, and modulation.
2. Description of the Related Art
FIG. 1 is a plan view showing a structure of an electrode formed on a semiconductor surface in a high-frequency FET in common use. In this high-frequency semiconductor device, three source electrodes 2 extend from a source pad section 1 and two drain electrodes 4 extending from a drain pad section 3 are disposed between the source electrodes 2. Four very long, narrow gate electrodes 6 extending from two gate pad sections 5 disposed between the tip of the drain electrodes 4 and the source pad section 1 extend in areas sandwiched by the source electrodes 2 and the drain electrodes 4. In other words, this high-frequency semiconductor device is a horizontal-type (plane-type) FET in which the source electrodes 2, the gate electrodes 6, and the drain electrodes 4 are formed on the same plane.
To make such an FET usable at higher frequencies, it is necessary to reduce the distance between the source electrode and the drain electrode and to narrow a gate electrode (reduce the gate length). A narrow and long gate electrode has large parasitic resistance and large, parasitic capacitance, however, and thereby the characteristics deteriorate by noise increase, operating-frequency decrease, gain reduction, and increased input/output reflection loss. To form a narrow and long gate electrode, an advanced fine machining technology (especially advanced photolithography) is required. However, such technology increases variations in characteristics and reduces yield.
When the FET is viewed as a waveguide which transmits a micro wave, it has very unnatural structure and its operating range is limited to a low frequency zone in which the FET can be approximated to a lumped-constant circuit device.
FIG. 2 is a plan view illustrating an electrode structure of an air-bridge-gate-structure FET which improves the above describes characteristics deterioration. In this structure, a source electrode 8 disposed between source pad sections 7 on a semiconductor surface faces a drain electrode 9 over its full length. A wide gate electrode 11 extends over the source electrode 8 from a gate pad section 10 disposed at the side opposite the drain electrode 9 against the source electrode 8, and the edge of the gate electrode 11 is Schottky-connected to the semiconductor surface between the source electrode 8 and the drain electrode 9.
Since the gate electrode 11 can be made wide in such an air-bridge-gate-structure FET, the parasitic resistance and the parasitic inductance of the gate electrode 11 are reduced and the RF characteristics (especially noise characteristics) are improved.
At a portion where the gate electrode 11 passes over the source electrode 8, a parasitic capacitor is generated between the source electrode 8 and the gate electrode 11, which decreases the operating frequency. To reduce this parasitic capacitance, the source electrode 8 needs to be narrowed. If the source electrode 8 is narrowed, since the source electrode 8 has additional parasitic resistance and additional parasitic inductance, characteristics improvement is limited in such a method. Also in this air-bridge-gate-structure FET, a fine machining technology for a gate electrode is not easily implemented in the same way as for a horizontal-type FET.
When this air-bridge-gate-structure FET is viewed as a waveguide, it has very unnatural structure like a horizontal-type FET, and its operating range is limited to a low frequency zone in which the FET can be approximated to a lumped-constant circuit device.
To solve the problem of characteristics deterioration of a high-frequency FET in millimetric-wave and quasi-millimetric-wave ranges, it is an important issue to improve precision in machining a gate electrode and to eliminate wiring resistors and parasitic components such as parasitic capacitors and parasitic inductors in the gate electrode and the drain electrode, as described above. It is very difficult, however, to suppress characteristics deterioration in a high-frequency range in a conventional FET structure, and a semiconductor device suited to a millimetric wave and a quasimillimetric wave cannot be manufactured.